Barrels for self-winding timepieces conventionally include a drum housing a mainspring coiled about a barrel arbour. To prevent any overwinding of the mainspring, which could cause it to give way or damage the gear train of the self-winding device, the mainspring is not fixed to the lateral wall of the drum, but friction coupled, via its last coil or turn, using an elastic strip called a slip spring. The friction coupling between the mainspring and the lateral wall of the drum must be calculated such that, beyond a defined maximum drive torque, the last coil slips along said wall and thus reduces the tension of the mainspring. The surface state of the internal lateral wall of the drum and the lubrication thereof are factors that greatly affect the operation of this friction coupling. Another determining factor concerns the geometry of the internal lateral wall.
According to the state of the art, the internal lateral wall of the drum includes friction surfaces, alternating with cut out portions in the form of notches in which the end of the mainspring abuts. The combined effects of friction and abutment of the last coil on the lateral wall enable high maximum drive torques to be attained and thus a large power reserve.
However, one difficulty linked to the presence of notches in the internal lateral wall of the drum is that of wear. The problem of wear is particularly inconvenient since it self-amplifies and may lead to significant damage, malfunctions or even to breakage of a part. When the friction of the last mainspring coil tears off fine particles from the wall, they make the friction more abrasive, which produces more particles and accelerates the process of wear. Over time, the friction coupling between the mainspring and the lateral wall of the barrel may be seriously affected.
It is known that the presence of notches in the internal lateral wall of the drum contributes to the process of barrel wear. Indeed, the notches form square salient edges at the upstream and downstream boundaries of the friction surfaces relative to the direction in which the last coil slips, i.e. respectively at the outlet and inlet of the notch. The mainspring pressure is very high at these square salient edges. They may therefore quickly become blunted.
According to a widespread prejudice among those skilled in the art, the square salient edges delimiting the friction surfaces upstream relative to the direction in which the last coil slips, are mainly responsible for generating, by wear, particles that accelerate the process of wear. Indeed, these edges play an active part in the friction coupling, since the end of the last coil abuts against the salient edges at the outlet of the notch, unlike the square salient edges at the inlet to the notch, which are virtually passive in the friction coupling.
Observations made within the scope of the present invention show that, contrary to commonly accepted belief, the square salient edges delimiting the friction surfaces downstream relative to the slip direction of the last coil, also play an important part as precursors, by wear, of the generation of particles accelerating the process of wear. The explanation for this is as follows: The slipping movement of the last coil against the friction surfaces is generally slow because of the permanent quasi-equilibrium between the drive torque and the friction torque. The end of the last mainspring coil thus arrives slowly at the notch inlet and rubs against the square salient edge which demarcates the downstream friction surface, at a low speed. At low speed, the lubrication efficiency is mediocre and there is significant friction. There is therefore significant wear of the square salient edge at the notch inlet. The situation is slightly different as regards the square salient edge which demarcates the friction surface upstream relative to the slip direction of the last coil. Indeed, the end of the last mainspring coil abuts against this square salient edge at the notch outlet, which substantially increases the mainspring tension up to the defined maximum drive torque. When the winding of the mainspring increases beyond this value, the end of the mainspring leaves the notch, abruptly breaking the quasi-equilibrium between the drive torque and the friction torque. The notch outlet then wears very quickly. At a high speed, lubrication efficiency is good, which greatly limits friction. Ultimately, the wear of the square salient edges both at the inlet and outlet of the notch play an important part in the degradation of the surface by wear, even if the inlet edges do not actively participate as stop members in the friction coupling between the last mainspring coil and the internal lateral drum wall.